The Hare Krishna Movement

…Oṁkāra, or the praṇava, is the seed of transcendental realization, and it is composed of the three transcendental letters a-u-m. By its chanting by the mind, in conjunction with the breathing process, which is a transcendental but mechanical way of getting into trance, as devised by the experience of great mystics, one is able to bring the mind, which is materially absorbed, under control.

In the Bhagavad-gītā, the praṇava (oṁkāra) has been accepted as the direct, literal representation of the Supreme Absolute Truth. One who is not able to chant directly the holy name of the Lord, as recommended above, can easily chant the praṇava (oṁkāra). This oṁkāra is a note of address, such as “O my Lord,” just as oṁ hari om means “O my Lord, the Supreme Personality of Godhead.” As we have explained before, the Lord’s holy name is identical with the Lord Himself. So also is oṁkāra. But persons who are unable to realize the transcendental personal form or name of the Lord on account of their imperfect senses (in other words, the neophytes) are trained to the practice of self-realization by this mechanical process of regulating the breathing function and simultaneously repeating the praṇava (oṁkāra) within the mind.

Srimad Bhagavatam
By His Divine Grace A.C. Bhaktivedanta Swami Prabhupada
Canto 2, Chapter 1, Text 17

The First Step in God Realization

abhyasen manasā śuddhaṁ
trivṛd-brahmākṣaraṁ param
mano yacchej jita-śvāso
brahma-bījam avismaran

abhyaset—one should practice; manasā—by the mind; śuddham—sacred; tri-vṛt—composed of the three; brahma-akṣaram—transcendental letters; param—the supreme; manaḥ—mind; yacchet—get under control; jita-śvāsaḥ—by regulating the breathing air; brahma—absolute; bījam—seed; avismaran—without being forgotten.

After sitting in the above manner, make the mind remember the three transcendental letters [a-u-m], and by regulating the breathing process, control the mind so as not to forget the transcendental seed.

Purport
Oṁkāra, or the praṇava, is the seed of transcendental realization, and it is composed of the three transcendental letters a-u-m. By its chanting by the mind, in conjunction with the breathing process, which is a transcendental but mechanical way of getting into trance, as devised by the experience of great mystics, one is able to bring the mind, which is materially absorbed, under control. This is the way of changing the habit of the mind. The mind is not to be killed. Mind or desire cannot be stopped, but to develop a desire to function for spiritual realization, the quality of engagement by the mind has to be changed. The mind is the pivot of the active sense organs, and as such if the quality of thinking, feeling and willing is changed, naturally the quality of actions by the instrumental senses will also change. Oṁkāra is the seed of all transcendental sound and it is only the transcendental sound which can bring about the desired change of the mind and the senses. Even a mentally deranged man can be cured by treatment of transcendental sound. In the Bhagavad-gītā, the praṇava (oṁkāra) has been accepted as the direct, literal representation of the Supreme Absolute Truth. One who is not able to chant directly the holy name of the Lord, as recommended above, can easily chant the praṇava (oṁkāra). This oṁkāra is a note of address, such as “O my Lord,” just as oṁ hari om means “O my Lord, the Supreme Personality of Godhead.” As we have explained before, the Lord’s holy name is identical with the Lord Himself. So also is oṁkāra. But persons who are unable to realize the transcendental personal form or name of the Lord on account of their imperfect senses (in other words, the neophytes) are trained to the practice of self-realization by this mechanical process of regulating the breathing function and simultaneously repeating the praṇava (oṁkāra) within the mind. As we have several times expressed, since the transcendental name, form, attributes, pastimes, etc., of the Personality of Godhead are impossible to understand with the present material senses, it is necessary that through the mind, the center of sensual activities, such transcendental realization be set into motion. The devotees directly fix their minds on the Person of the Absolute Truth. But one who is unable to accommodate such personal features of the Absolute is disciplined in impersonality to train the mind to make further progress.

Also from;

Bhagavad-gita As It Is
By His Divine Grace A.C. Bhaktivedanta Swami Prabhupada
Verses 8.13, 7.8

(Bg. 8.13)
oṁ ity ekākṣaraṁ brahma
vyāharan mām anusmaran
yaḥ prayāti tyajan dehaṁ
sa yāti paramāṁ gatim

om—the combination of letters, omkāra; iti—thus; eka-akṣaram—supreme, indestructible; brahma—absolute; vyāharan—vibrating; mām—Me (Kṛṣṇa); anusmaran—remembering; yaḥ—anyone; prayāti—leaves; tyajan—quitting; deham—this body; saḥ—he; yāti—achieves; paramām—supreme; gatim—destination.

After being situated in this yoga practice and vibrating the sacred syllable om, the supreme combination of letters, if one thinks of the Supreme Personality of Godhead and quits his body, he will certainly reach the spiritual planets.

Purport
It is clearly stated here that om, Brahman, and Lord Kṛṣṇa are not different. The impersonal sound of Kṛṣṇa is om, but the sound Hare Kṛṣṇa contains om. It is clearly recommended in this age that if one quits his body at the end of this life chanting the mahāmantra, Hare Kṛṣṇa, he will reach the spiritual planets. Similarly, those who are devotees of Kṛṣṇa enter the Kṛṣṇa planet or Goloka Vṛndāvana, whereas the impersonalists remain in the brahmajyoti. The personalists also enter many innumerable planets in the spiritual sky known as Vaikuṇṭhas.

(Bg. 7.8)
raso ‘ham apsu kaunteya
prabhāsmi śaśi-sūryayoḥ
praṇavaḥ sarva-vedeṣu
śabdaḥ khe pauruṣaṁ nṛṣu

rasaḥ—taste; aham—I; apsu—in water; kaunteya—O son of Kuntī; prabhā asmi—I am the light; śaśi-sūryayoḥ—in the sun and the moon; praṇavaḥ—the three letters A.U.M.; sarva—in all; vedeṣu—in the Vedas; śabdaḥ—sound vibration; khe—in the ether; pauruṣam—ability; nṛṣu—in man.

O son of Kuntī [Arjuna], I am the taste of water, the light of the sun and the moon, the syllable om in the Vedic mantras; I am the sound in ether and ability in man.

Purport
This verse explains how the Lord is all-pervasive by His diverse material and spiritual energies. The Supreme Lord can be preliminarily perceived by His different energies, and in this way He is realized impersonally. As the demigod in the sun is a person and is perceived by his all-pervading energy, the sunshine, similarly, the Lord, although in His eternal abode, is perceived by His all-pervading, diffusive energies. The taste of water is the active principle of water. No one likes to drink sea water because the pure taste of water is mixed with salt. Attraction for water depends on the purity of the taste, and this pure taste is one of the energies of the Lord. The impersonalist perceives the presence of the Lord in water by its taste, and the personalist also glorifies the Lord for His kindly supplying water to quench man’s thirst. That is the way of perceiving the Supreme. Practically speaking, there is no conflict between personalism and impersonalism. One who knows God knows that the impersonal conception and personal conception are simultaneously present in everything and that there is no contradiction. Therefore Lord Caitanya established His sublime doctrine: acintya-bheda and abheda-tattvam–simultaneously one and different.

The light of the sun and the moon is also originally emanating from the brahmajyoti, which is the impersonal effulgence of the Lord. Similarly praṇava or the omkāra transcendental sound used in the beginning of every Vedic hymn to address the Supreme Lord also emanates from Him. Because the impersonalists are very much afraid of addressing the Supreme Lord Kṛṣṇa by His innumerable names, they prefer to vibrate the transcendental sound omkāra. But they do not realize that omkāra is the sound representation of Kṛṣṇa. The jurisdiction of Kṛṣṇa consciousness extends everywhere, and one who knows Kṛṣṇa consciousness is blessed. Those who do not know Kṛṣṇa are in illusion, and so knowledge of Kṛṣṇa is liberation, and ignorance of Him is bondage.